c-Src plays a major role in the growth, progression and metastasis of a large number of cancers. c-Src can be the transforming element of the oncogenic Rous sarcoma retrovirus. Subsequently, it has been demonstrated that c-Src kinase can have the oncogenic potential. Gene knockout experiments suggest that inhibition of some members of the Src family might have potential therapeutic benefit.
Tyrosine kinases (TKs) phosphorylate tyrosine residues in peptides and proteins. These enzymes are key elements in the regulation of cell signaling including cell proliferation and cell differentiation. Protein TKs comprise the receptor TKs, including the epidermal growth family members (HER1 and HER2 for example), platelet derived growth factor (PDGF) and kinases that play a role in angiogenesis (Tie-2 and KDR for example), and the cellular or non-receptor kinases, which include members of the Src family.
c-Src TK is one of three members of the Src family expressed ubiquitously. c-Src is expressed at low levels in most cell types and, in the absence of the appropriate extracellular stimuli, maintained in an inactive conformation through phosphorylation of a regulatory tyrosine domain at Tyr530. Activation of c-Src occurs through dephosphorylation of the Tyr530 site and phosphorylation of a second tyrosine, Tyr419, present in the kinase domain of the enzyme.
Src kinase modulates signal transduction through multiple oncogenic pathways, including EGFR, HER2, PDGFR, FGFR and VEGFR. Thus, blocking signaling through the inhibition of the kinase activity of Src can be an effective means of modulating aberrant pathways that drive the oncogenic transformation of cells.
There exists a body of evidence of misregulated increased kinase activity of c-Src in several human tumor types, most notably colon and breast tumors. Misregulated c-Src TK activity has also been associated with adhesion and cytoskeletal changes both in tumor cells and otherwise, ultimately resulting in an invasive phenotype that may be motile. c-Src TK activity has been shown to be an important component in the epithelial to mesenchymal transition that occurs in the early stages of invasion of carcinoma cells. c-Src activity is also known to be essential in the turnover of local adhesions, a critical cell-motility component. In in vivo models of metastases, c-Src inhibition markedly reduces the rate of lymph and liver metastases. Clinical data supports the link between misregulated Src activity and the increased invasive potential of tumor cells. In colon tumors, increased c-Src TK activity has been shown to correlate to tumor progression, with the highest activity found in metastatic tissue. Increased Src activity in colon tumors might be an indicator of poor prognosis. In breast and ovarian cancers, enhancement of Src kinase activity has been reported, and in transitional cell carcinoma of the bladder, c-Src activity peaked as superficial tumors became muscle invasive.
Biochemically, cellular stimuli that lead to Src activation result in increased association between Src and the cytoskeleton. As a result, Src mediates the phosphorylation of many intracellular substrates such as EGFR, FAK, PYK2, paxillin, Stat3, and cyclin D. The biological effects of these interactions affect cell motility, adhesion, cell cycle progression, and apoptosis and might have some connection to the disease related effects stated above. Thus, Src plays a role in responses to regional hypoxia, limited nutrients, and internal cellular effects to self-destruct.
Increased c-Src TK activity results in breakdown of the E-cadherin-mediated epithelial cell-cell adhesion, which can be restored by Src inhibition. Intimate connections between increased VEGF activity, Src activity, and cellular barrier function related to vascular leak have been also demonstrated. Inhibition of Src results in decrease in vascular leak when exogenous VEGF is administered in in vivo studies. Examples where excessive vascular permeability leads to particularly deleterious effects include pulmonary edema, cerebral edema, and cardiac edema.
The cascade of events leading to loss of endothelial barrier function is complex and incompletely understood. Data support some role for kinases in this process. For example, VEGF-mediated edema has been shown to involve intracellular signaling by Src family kinases, protein kinase C, and Akt kinase. Rho-associated kinases have been linked to thrombin-mediated vascular leakage, and protein kinase C to TNF-induced leakage. Kinases are believed to mediate the phosphorylation of junctional proteins such as beta-catenin and vascular endothelial VE-cadherin, leading to the dissolution of adherens junctions and the dissociation of cadherin-catenin complexes from their cytoskeletal anchors. Proteins which regulate the intercellular contractile machinery such as myosin light chain kinase (MLCK) and myosin light chain (MLC) are also activated, resulting in cellular contraction, and therefore an opening of intercellular junctions.
A general approach to the inhibition of vascular leakage can be to interfere with any of the underlying mechanistic pathways, whether by inhibition of kinase signaling or the intercellular contractile apparatus or other cellular processes. This can then lead to potential treatments for edema and its associated pathologies. For example, inhibiting edema formation should be beneficial to overall patient outcome in situations such as inflammation, allergic diseases, cancer, cerebral stroke, myocardial infarction, pulmonary and cardiac insufficiency, renal failure, and retinopathies, to name a few. Furthermore, as edema is a general consequence of tissue hypoxia, it can also be concluded that inhibition of vascular leakage represents a potential approach to the treatment of tissue hypoxia. For example, interruption of blood flow by pathologic conditions (such as thrombus formation) or medical intervention (such as cardioplegia, organ transplantation, and angioplasty) could be treated both acutely and prophylactically using inhibitors of vascular leakage, especially as in the case of Src inhibitors.
Accordingly, a small molecule inhibitor of c-Src can be beneficial for the treatment of several disease states.